1. Field of the Invention
This invention relates to an exposure apparatus for printing the pattern of a mask on a semi-conductive wafer, for example, in the manufacturing process of semiconductor integrated circuits, and in particular to a projection exposure apparatus utilizing an excimer laser.
2. Related Background Art
A projection exposure apparatus utilizing the output of an excimer laser to transfer the pattern of a mask or reticle (hereinafter referred to as the mask) onto a semicondcutive wafer is known from U.S. Pat. No. 4,458,994.
It is known that the output wavelength of an excimer laser source can be determined variously by the selection of excimer gas, and it is suggested in the aforementioned U.S. patent that when using the output of an excimer laser source as the exposure energy in a lithography system utilizing a projection optical system, it is relatively easy to design the projection optical system so as to be corrected in chromatic aberration relative to the bandwidth of the output wavelength thereof. That is, where the projection optical system is constructed by the use of a plurality of optical elements differing in refractive index, it is possible to correct chromatic aberration relative to a wavelength of a limited bandwidth.
However, in order to enhance the transmittance of the projection optical system, it is preferable that all elements constituting the refracting system of the optical system be formed of quartz. In that case, it is difficult to correct chromatic aberration completely relative to a plurality of wavelengths and therefore, only a single wavelength determined in design must be used as the exposure wavelength.
Accordingly, for such a reason, in an exposure apparatus provided with an excimer laser source, the stability of the wavelength of the laser beam output from the laser source (the stability of the central wavelength and the wavelength range) becomes an important matter in respect of the fluctuation of the projection magnification, the fluctuation of the focus position, etc. Even slight deviation of the wavelength causes significant fluctuation of the projection magnification, fluctuation of the focus position, etc.